Impact ofMycoplasma ovipneumoniaeon juvenile bighorn sheep (Ovis canadensis) survival in the northern Basin and Range ecosystem

Abstract

Determining the demographic impacts of wildlife disease is complex because extrinsic and intrinsic drivers of survival, reproduction, body condition, and other factors that may interact with disease vary widely. Mycoplasma ovipneumoniae infection has been linked to persistent mortality in juvenile bighorn sheep (Ovis canadensis), although mortality appears to vary widely across subspecies, populations, and outbreaks. Hypotheses for that variation range from interactions with nutrition, population density, genetic variation in the pathogen, genetic variation in the host, and other factors. We investigated factors related to survival of juvenile bighorn sheep in reestablished populations in the northern Basin and Range ecosystem, managed as the formerly-recognized California subspecies (hereafter, “California lineage”). We investigated whether survival probability of 4-month juveniles would vary by (1) presence of M. ovipneumoniae-infected or exposed individuals in populations, (2) population genetic diversity, and (3) an index of forage suitability. We monitored 121 juveniles across a 3-year period in 13 populations in southeastern Oregon and northern Nevada. We observed each juvenile and GPS-collared mother semi-monthly and established 4-month capture histories for the juvenile to estimate survival. All collared adult females were PCR-tested at least once for M. ovipneumoniae infection. The presence of M. ovipneumoniae-infected juveniles was determined by observing juvenile behavior and PCR-testing dead juveniles. We used a known-fate model with different time effects to determine if the probability of survival to 4 months varied temporally or was influenced by disease or other factors. We detected dead juveniles infected with M. ovipneumoniae in only two populations. Derived juvenile survival probability at four months in populations where infected juveniles were not detected was more than 20 times higher. Detection of infected adults or adults with antibody levels suggesting prior exposure was less predictive of juvenile survival. Survival varied temporally but was not strongly influenced by population genetic diversity or nutrition, although genetic diversity within most study area populations was very low. We conclude that the presence of M. ovipneumoniae can cause extremely low juvenile survival probability in translocated bighorn populations of the California lineage, but found little influence that genetic diversity or nutrition affect juvenile survival. Yet, after the PCR+ adult female in one population died, subsequent observations found 11 of 14 ( 79%) collared adult females had surviving juveniles at 4-months, suggesting that targeted removals of infected adults should be evaluated as a management strategy.

Modelling management strategies for chronic disease in wildlife: Predictions for the control of respiratory disease in bighorn sheep

“…Figure 4c,d), or increased carrying capacity as defined in the model (as in the case of range expansion, Figure 4i). This is consistent with experimental and observational field studies reporting marked improvement in recruitment following complete clearance of M. ovipneumoniae among ewes (Garwood et al, 2020;Spaan et al, 2021). When depopulation-and-reintroduction efforts succeeded at removing all infected individuals, they had the added benefit of boosting the post-management population size, further accelerating population resurgence.…”

Natural history of a bighorn sheep pneumonia epizootic: Source of infection, course of disease, and pathogen clearance

“…Spaan et al. (2021) have reported a similar population response to natural death of a single carrier ewe. The relatively short duration of postepizootic, pneumonia‐induced mortality in NBR lambs could be due to several factors, including the low number of survivors, fitness costs associated with carriage (Dekelaita et al., 2020), or the high virulence of this epizootic selecting against animals less likely to resist infection and thus become carriers.…”

“…Brief to prolonged periods of low recruitment following all age epizootics due to pneumonia epizootics in young of the year is characteristic of this disease. This signature feature is thought to be caused by the presence of persistent infected but largely asymptomatic carriers that transmit M. ovipneumoniae to susceptible lambs (Cassirer et al., 2018; Garwood et al., 2020; Spaan et al., 2021). Postmortem testing of lambs in 2018 confirmed M. ovipneumoniae‐ associated pneumonia, and death of the single known carrier ewe in spring 2019 prior to the seasonal onset of parturition coincided with the first pneumonia‐free (and serologically M. ovipneumoniae exposure‐free) cohort of lambs, suggesting that the presence of one carrier female was associated with the lamb pneumonia epizootic in 2018.…”

Restoration of bighorn sheep: History, successes, and remaining conservation issues

“…Modeling of M. ovipneumoniae disease dynamics indicates that depopulation and restoration can assist recovery following an epidemic (Almberg et al, 2022). Indeed, following rapid depopulation of bighorn sheep in the Montana Mountains of northwestern Nevada, United States, in 2015 after detection of a new outbreak of M. ovipneumoniae with high mortality, that strain was not detected in subsequent monitoring of the nearby Trout Creek metapopulation of bighorn in southeastern Oregon (Spaan, 2022). Depopulation is likely considered more readily in systems of restored rather than native populations.…”